1. Field of the Invention
The present invention relates to a receptacle module for connecting optical fiber cables.
Specifically, the present invention relates to a receptacle module into which an optical module provided with a light emitting element and/or a light receiving element is fixed by insertion and to a receptacle module into which an optical fiber connector is inserted to optically connect an optical fiber cable of an optical module and an optical fiber cable of the optical fiber connector.
2. Description of the Related Art
Known in the art is an optical fiber connection assembly obtained by positioning two optical fiber connectors to face each other and causing the end faces of the optical fiber cables to come into facial contact.
An example of such an optical fiber connection assembly will be explained with reference to FIG. 1. The optical fiber connection assembly illustrated in FIG. 1 is comprised of an adapter 2 and optical fiber connectors 1a and 1b as examples of MT-RJ connectors. The optical fiber connectors 1a and 1b are engaged by using the adapter 2 to bring the end faces of optical fiber cables 30a and 30b passing through these optical fiber connector 1a and 1b into facial contact with each other and thereby optically connect the optical fiber cables 30a and 30b. 
The optical fiber cable 30a, as illustrated in FIG. 3B, is provided with a plurality of, for example, four, optical fibers. FIG. 1 illustrates only one optical fiber among them.
The adapter 2 has a first (left side) sleeve 10a for receiving the optical fiber connector 1a and a second (right side) sleeve 10b for receiving the optical fiber connector 1b. The first sleeve 10a and the second sleeve 10b are formed integrally. The adapter 2 is formed is with a window 10c with an opening for communicating the two sleeves 10a and 10b. A right front end of a left side ferrule 22a explained later and a left front end of a right side ferrule 22b are inserted into the opening of the window 10c so as to achieve stable positioning of the end faces of the optical fiber cables exposed at the end faces of these ferrules 22a and 22b at the position of this window 10c. 
An engagement use opening 10a1 extending from the inside to the outside of the sleeve 10a is formed in the wall of the sleeve 10a. When a latch 20a1 of a housing 20a is inserted in the engagement use opening 10a1, the optical fiber connector 1a held in the sleeve 10a is fixed in place.
The optical fiber connector 1a partially inserted in the inside space of the first sleeve 10a has the housing 20a, a spring 21a, and the ferrule 22a. 
The housing 20a is shaped as a hollow cylinder with a bottom. The spring 21a and part of the ferrule 22a are contained in the hollow cylinder of the housing 20a. The latch 20a1 and a guard shaped pressing portion 20a2 are provided on the outer wall of the housing 20a along an axial direction.
The ferrule 22a is comprised of a middle large area part and small area parts on its two sides. The left small area part and the middle large area part are contained in the hollow cylinder of the housing 20a, but the right small area part is passes through the opening of the housing 20a and is inserted into the opening of the window 10c. 
The ferrule 22a is pressed to the right side by the spring 21a contained in the inside space of the housing 20a, but the middle large area part abuts against the opening of the housing 20a and stops it there. Conversely, when a leftward pressing force is added to the right small area part, the ferrule 22a moves to the left side, but stops at a position balanced with the force of the spring 21a. 
The optical fiber cable 30a passes through the inside of the housing 20a and is exposed at the end face of the right small area part of the ferrule 22a. The other optical fiber cable 30b also posses through the inside of the housing 20b and is exposed at the end face of the left small area part of the ferrule 22b. 
The optical fiber connector 1b is structured the same as the optical fiber connector 1a. 
The end face of the right small area part of the ferrule 22a of the optical fiber connector 1a and the end face of the left small area part of the ferrule 22b of the optical fiber connector 1b are polished to a fiber protrusion so that they may be accurately physical connected. For example, a diameter of the mode field of the single mode optical fiber is about 10 xcexcm. Therefore, it is necessary to very precisely process these end faces so as to prevent deterioration in the resistance to insertion loss and other various characteristics when connecting these optical fiber cables.
When the housing 20a is pushed into the sleeve 10a until the guard shape pressing portion 20a2 abuts against the front end of the sleeve 10a and the latch 20a1 is fits in the engagement use opening 10a1 of the sleeve 10a, the optical fiber connector 1a is inserted in the sleeve 10a. 
The optical fiber connector 1b is similarly inserted in the sleeve 10b. 
At this time, the right end face of the ferrule 22a and the left end face of the ferrule 22b abut against each other at an optical reference plane 40 at the substantial center of the window 10c. Due to this connection, the spring 21a contained in the inside space of the housing 20a and the spring 21b contained in the inside space of the housing 20b are compressed. The spring 21a and the spring 21b are manufactured to substantially same characteristics and uniformly press against the end faces of the ferrule 22a and the ferrule 22b which they abut against so as to maintain the connection of the right end face of the ferrule 22a and the left end face of the ferrule 22b. 
The right end face of the ferrule 22a and the front end of the optical fiber cable 30a exposed there are polished to a fiber protrusion. Similarly, the left end face of the ferrule 22b and the front end of the optical fiber cable 30b exposed there are polished to a fiber protrusion. Therefore, when the right end face of the ferrule 22a and the left end face of the ferrule 22b are connected, the end face of the optical fiber cable 30a and the end face of the optical fiber cable 30b directly contact each other and therefore the optical fiber cable 30a and the optical fiber cable 30b are optically connected.
The connection load (stress) of the connecting part of the optical fiber cables 30a and 30b is defined by certain standards, for example, the Japan Industrial Standard (JIS standard). For example, in the case of a fiber transmission system optical fiber connector (FC) and a single fiber coupling connector (SC), it is prescribed that the load be 7.8 to 11.8N, while in the case of a multiple fiber push on connector (MPO), it is prescribed that the load be 6.8 to 11.8N.
In addition to optical fiber connection assemblies connecting optical fiber connectors 1a and 1b explained with reference to FIG. 1, direct connection of the optical module fixed in the receptacle and the optical fiber connector has been demanded. The principle is basically the same as the optical fiber connection assembly explained with reference to FIG. 1.
The combination of a receptacle and optical module is referred to as a xe2x80x9creceptacle modulexe2x80x9d in the present specification.
A novel structure of a receptacle module enabling stable optical connection of the optical fiber cable of an optical fiber connector and the optical fiber cable of an optical module is being sought. The connection load for connection with an optical fiber cable in an optical module when inserting an MT-RJ connector in a receptacle module has not yet been stipulated, but a connection load of 7.8 to 11.8N is being considered.
An object of the present invention is to provide a receptacle module of a novel structure enabling stable optical connection between an optical fiber cable of an optical fiber connector and an optical fiber cable of an optical module.
Another object of the present invention is to provide a receptacle module substantially free of deviation of optical positions of optically connected optical fiber cables and with little deterioration of characteristics of optical signals transmitted through the optical fiber cables even when an external force, vibration, or the like is applied to the receptacle module in a direction orthogonal to the axial direction in a state where the optical fiber connector is attached in the receptacle of the receptacle module.
According to a first aspect of the present invention, there is provided a receptacle module having a receptacle having first and second sleeves formed at positions facing each other in an axial direction by way of a window and an optical module provided with at least one of a light receiving element and/or a light emitting element and a first optical fiber and having a projection for insertion through the window and a body for insertion into the second sleeve, the first optical fiber being exposed at an end face of the projection, wherein a length of the projection is formed to be substantially at least a length of the window in the axial direction, the end face of the projection positioned at an end face of the window at the first sleeve side or is positioned inside the first sleeve when the body of the optical module is fully inserted into the second sleeve, and the first sleeve receives an optical fiber connector having an the end face at which is exposed a second optical fiber is exposed for optical connection with the first optical fiber.
Preferably, the surface of the first optical fiber exposed at the end face of the projection of the optical module is polished so as to connect with the second optical fiber by physical contact.
More preferably, a connection load of at least 1.09 times the standard value of the connection load when optically connect two optical fiber connectors is applied to a connecting part of the first optical fiber and the second optical fiber when the optical fiber connector is inserted in the first sleeve.
Still more preferably, a connection load of at least 8.5N is applied.
Preferably, the optical fiber connector is one having a ferrule through which the second optical fiber passes and is exposed at its end face, a housing holding the ferrule to be able to move in the axial direction, and a spring for pushing the ferrule in the housing toward the inside of the first sleeve.
More preferably, the optical fiber connector is selected from an MT-RJ, MPO, SC, FC and MPX connectors.
Preferably, the ferrule of the optical fiber connector is moved at least 0.5 mm at the end face of the projection of the optical module when the optical fiber connector is inserted in the first sleeve.
According to a second aspect of the present invention, there is provided an optical fiber connection assembly having a receptacle having first and second sleeves formed at positions facing each other in an axial direction by way of a window; an optical module provided with at least one of a light receiving element and/or a light emitting element and a first optical fiber and having a projection for insertion through the window and a body for insertion into the second sleeve, the first optical fiber being exposed at an end face of the projection; and an optical fiber connector having a ferrule through which a second optical fiber cable passes and is exposed at its end face, a housing holding the ferrule to be able to move in the axial direction, and a spring for pushing the ferrule in the housing toward the inside of the first sleeve; wherein a length of the projection is formed to be substantially at least a length of the window in the axial direction; the end face of the projection positioned at an end face of the window at the first sleeve side or is positioned inside the first sleeve when the body of the optical module is fully inserted into the second sleeve; and the end faces of the first optical fiber and the second optical fiber are optically connected by a connection load of at least 1.09 times the standard value of the connection load when optically connect two optical fiber connectors when the optical fiber connector is inserted in the first sleeve.
According to a third aspect of the present invention, there is provided a receptacle module comprising a receptacle having first and second sleeves spatially connected with each other and an optical module provided with at least one of a light receiving element and a light emitting element and an optical fiber, the optical fiber being exposed at an end face of the optical module and the optical module being fixed in the first sleeve of the receptacle; wherein the second sleeve of the receptacle receiving an optical fiber connector, an optical fiber being exposed at an end face of the optical fiber connector; and the optical module and the optical fiber connector being fixed with a load of at least 8.5N at their end faces.
According to a fourth aspect of the present invention, there is provided a receptacle module comprising a receptacle having first and second sleeves spatially connected with each other and an optical module provided with at least one of a light receiving element and a light emitting element and an optical fiber, the optical fiber being exposed at an end face of the optical module and the optical module being fixed in the first sleeve of the receptacle; wherein the second sleeve of the receptacle receiving an optical fiber connector, an optical fiber cable being exposed at an end face of the optical fiber connector; and the optical module and the optical fiber connector being fixed with a load of at least 1.09 times the lower limit of the standard load for connection of two optical fiber connectors.
According to a fifth aspect of the present invention, there is provided a method for selecting a connection load applied to a connecting part of an optical module and an optical fiber connector having optical connectors to connect each other by physical contact, comprising: the connection load is selected as a value of at least 1.09 times the standard minimum value of the connection load when optically connect two optical fiber connectors.